1. Field of the Invention
The present invention relates to a method for producing a base of a catalyst carrier used for an exhaust gas purification device in an automobile.
2. Description of the Prior Art
Since the nineteen-seventies, air pollution by automobile emission control has become a social problem, and regulations for the automobile exhaust emissions have been promulgated to the effect that passenger automobiles must be equipped with a device for purifying the exhaust gas. Several systems have been proposed for devices for purifying the exhaust gas, but the most widely used at present is a catalyst converter system whereby HC and CO are oxidized and, simultaneously, NOx is reduced. These catalyst converters are made up of a ceramic honeycomb of corderite, mounted in a metallic cylinder, as a fundamental body, on which porous .gamma.-Al.sub.2 O.sub.3 powder, impregnated with a noble metal (Pt.about.Pd) catalyst, is deposited. The ceramic honeycomb, however, has disadvantages in that it is not highly resistant to mechanical impact, and in addition, the exhaust resistance is undesirably high. The techniques disclosed to cope with these drawbacks include a known metal honeycomb in which metal foils of several tens in the form of a flat sheet and corrugated one are coiled into a roll insert mounted in an outer case of stainless steel.
Japanese Examined Patent Publication No. 57-55,886, discloses an improvement of the above method by a technique in which the metal honeycomb is fixed to the outer case at the ends of the body by electron beam welding or by brazing, or improved by the technique disclosed in Japanese Examined Patent Publication No. 58-23,138. These techniques, however, have various drawbacks. For example, a high Al-containing stainless steel used for the foil because of its excellent oxidation resistance has a poor rollability and is expensive. Also, from the viewpoint of the rollability, limitations should be imposed on the amount of alloying elements, which makes it difficult to attain a stable formation of an Al.sub.2 O.sub.3 film necessary for maintaining a sufficient oxidation resistance for a long time. In attempts to eliminate these drawbacks, Japanese Unexamined Patent Publication No. 54-97,593, Japanese Examined Patent Publication No. 57-3,418, Japanese Unexamined Patent Publication No. 54-33,888, and German Patent No. 2,745,188 disclose giving iron, stainless steel, heat-resistant alloy or the like, an Al coating, and then carrying out a chemical treatment or heat treatment in an oxidizing atmosphere, thereby forming an Al.sub.2 O.sub.3 film or alloy.
In the manufacture of a metal substrate, the metal foils are formed into a honeycomb, followed by a bonding process between corrugated sheets and flat sheets as well as bonding between the outer cylinder and honeycomb. Generally, this bonding is carried out by brazing. The brazing material used for this bonding must be a high Ni-brazing material with a high melting point and a high heat resistance, considering the severe condition under which the brazed assembly is used. Thus, Ni is oxidized at the surface of brazed parts by the exhaust gas. Since the Ni oxide has an effect of catalysis on unburnt components in the exhaust gas, a reaction takes place at the brazed parts, causing a partial rise in temperature at these parts. Accordingly, the brazed parts may melt down, if the Ni-brazing material has a melting point of less than 1100.degree. C. Accordingly, it is necessary to use a brazing material having a melting point of 1100.degree. C or more. Therefore, the heat treatment must be carried out at a temperature of 1100.degree. C or more, in vacuum, for a perfect brazing.
In addition, because of a high Al content of the substrate, the wettability and the flowability of the brazing material are greatly reduced on the substrate surface. Accordingly, the high temperature necessary for the heat treatment lowers the production efficiency and increases costs. Furthermore, the brazed parts of a honeycomb manufactured as described above contain only a small amount of Al diffused from the substrate material, with the result that an Al.sub.2 O.sub.3 film having an excellent heat-resistance cannot be formed. Thus, the heatresistance of the brazed parts is lower than that of the substrate material, and the adhesion of .gamma.-Al.sub.2 O.sub.3 powder is poor.
The catalyst carrier is exposed to severe operating conditions, for example, a high speed exhaust gas flow and extreme mechanical vibration upon acceleration. Therefore, the bonding strength of a carrier is another important factor in the durability of the device. As is clear from the above explanation, to enable the use of Al-coated material for the substrate of a catalyst carrier, a novel bonding technique must be developed.